1. Field of the Invention
The present invention relates to surface treatment of a metal, in particular to surface treatment of a metal using a phosphate chemical treatment film and, more particularly, to a method for efficiently recovering useful components of a treatment bath (aqueous solution) that can be used to form a film.
2. Description of the Related Art
Conventionally, non-electrolytic phosphate chemical treatment has been widely used as a method for surface treatment of a metal. A non-electrolytic phosphate chemical treatment bath contains, (i) phosphate ions and phosphoric acid, (ii) metal ions that form phosphate crystals and thus provide a film (zinc, manganese, iron, etc.), (iii) metal ions that are reduced from cations in a solution and precipitate as metal to form a film (nickel, copper, etc.), (iv) ions that are involved in the reaction in which aforementioned dissolved phosphoric acid and metal ions react to form a film (nitrate ion, etc.), and (v) metal ions that are not components of the film (sodium, etc.).
Although these chemical component ions are useful for carrying out the non-electrolytic phosphate chemical treatment reaction, they also contain component which may influence the environment. Therefore, waste water containing these ions is usually discharged after being subjected to waste water treatment (ion removal treatment). Waste water treatment equipment is necessary for this removal, and cost of operating this equipment is high. In recent years, prevention of environmental pollution has been strongly called for, and the regulation of emission into environment has been expanded to include phosphorus (phosphoric acid) and nitrogen (nitrate ions), in addition to heavy metal ions (cations) of which the emission has already been strictly controlled. Emission of phosphorus and nitrogen into environment has led to eutrophication of rivers and sea areas to such an extent that the emission of these substances must also be strictly controlled. Removal of phosphorus and nitrogen from waste water requires new equipment for waste water treatment.
In the non-electrolytic phosphate chemical treatment, (i) component ions such as Na ion that are not involved in forming a film are also included to adjust the pH of the treatment bath to thereby adjust the dissociation state of phosphoric acid. In addition, (ii) a solid content such as sludge is formed in the treatment bath. (iii) Ions having low degree of dissociation such as H2PO4−, HNO2 are also included. Therefore, it has not been possible to concentrate and recover only useful component ions that are involved in forming a film from the aqueous solution in the washing water tank (in which chemical components of the treatment bath carried by the treated article are diluted) that follows the treatment bath, and to return them to the chemical treatment bath in the form of a solution for reuse. This is because cations such as Na ions, that are not components for forming a film, are also concentrated at the same time, and in order to reuse the recovered solution, these ions, such as Na ions that are not components for forming a film, must be removed from the cation, and it is substantially impossible to selectively remove Na ion and the like from the cation.
The inventor of the present invention previously filed a patent relating to electrolytic phosphate chemical treatment (Japanese Unexamined Patent Publication No. 2000-234200). The phosphate chemical treatment bath of the invention does not contain cations (metal ions), such as Na ions, that are not involved in formation of a film. Therefore, when the above-mentioned aqueous solution in the washing water tank is concentrated, concentration of Na ion or the like does not take place.
In the present invention, an electrodialysis technique (apparatus) is used as the means for concentrating the aqueous solution in the washing water tank. Electrodialysis is a method in which electric voltage and current is applied to a solution to cause ions dissolved therein to pass through a membrane for electrodialysis so as to be separated, concentrated or diluted. Thus, an electrodialysis technique is a separation method that is directed only to a solution and ions dissolved in the solution.
However, a phosphate chemical treatment bath is used for a reaction between the components to form a film. Thus, as the reaction proceeds, the component ions dissolved in the solution are converted to a solid (film, sludge) or to a gas. In order to apply the electrodialysis technique to the phosphate chemical treatment bath and aqueous solution in the washing water tank, it is desirable to provide the solution in the treatment bath in a condition free of solid or gas, and if possible, to control the dissolution state (acid dissociation state, mobility, etc.) of various ions to a constant level.
However, no method is described in the above-mentioned Japanese Unexamined Patent Publication No. 2000-234200 for reliably removing these solids and gases from the treatment bath, and therefore, no information is disclosed for applying the electrodialysis technique to a phosphate chemical treatment bath and an aqueous solution in the washing water tank.
After intensive studies, the inventor of the present invention has found a phosphate chemical treatment method that is more appropriate in practice (Patent Application Nos. 2001-49017 and 2002-20568). In these inventions, a method is disclosed in which an electrolytic phosphate chemical treatment bath can be reliably maintained in the state of a solution.
It is an object of the present invention to provide a method for setting up an environment that permits an electrodialysis technique to be used in conjunction with a phosphate chemical treatment bath, and to provide means, utilizing this technique, which permit useful component ions in the electrolytic phosphate chemical treatment bath to be recovered and to be reused in situ, and which can also prevent the emission of nitrogen, phosphorus, and fluoride, from the phosphate chemical treatment bath, into the environment.